WO 2007/033825 shows a control of a group of glow plugs for a diesel engine. The glow plugs are periodically connected with supply lines according to pulse-width modulated signals. To provide the glow plugs with the required energy, the voltage drop over the supply lines is calculated by the help of the measured glow plug current. This calculation is done for each glow plug individually to control its temperature. The method is well adapted for ceramic glow plugs of which the resistance strongly varies over the temperature. On the other hand, this method uses a calculation based on a number of measurements and estimations including the risk that the control of the temperature is wrong.
It is accordingly at least one object of the invention to provide an alternative glow plug control unit that provides a more precise control of the temperature of the glow plugs. It is at least another object of the invention to provide a method for controlling a glow plug more precisely. In addition, other objects, desirable features, and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.
Embodiments of the invention provide a glow plug control unit that comprises a first switch for connecting a power supply node to a glow plug. The glow plug control unit further comprises a voltage measurement unit for measuring the voltage at the power supply lines. A current measurement unit is built for measuring the current through the first switch and a control circuit is built for controlling the first switch and, in a current control mode, for regulating the current through the first switch to a predefined value.
The resistance of metallic glow plugs is relatively stable at different temperature conditions compared to the resistance of ceramic glow plugs. The inventive glow plug control unit provides the current control mode in which the current through the glow plugs is regulated directly. The power in the glow plugs and the temperature is accordingly controlled by the help of the current measurement and the current control does not need to compensate the voltage drops. The compensation of the voltage drops needs a series of calculations which may be faulty because they are based on estimations and prior measurements of the resistance. The current control mode is used preferably for metallic glow plugs, as their resistance is relatively stable over temperature.
In an embodiment, the first switch comprises a transistor and the current measurement unit comprises a current mirror mirroring the current through the transistor of the first switch. A current mirror provides a direct measurement of the current through the first switch, which is equal to the current through the glow plug.
Preferably, the glow plug control unit comprises a second switch between the battery and the power supply node. This additional, second switch, may open and close the supply path between the battery and the glow plug. The second switch is a redundant to block the current flow independently of the status of the control circuit.
The current is also measured when the first switch is switched off. This makes it possible to check if no current flows through the first switch in the off-periods.
In an embodiment, the control circuit regulates the voltage at the glow plugs in a voltage control mode. This additional mode may preferably be used for ceramic glow plugs. The resistance of the ceramic glow plug depends strongly on the glow temperature. Accordingly, to calculate the power in the glow plugs, the voltage at the glow plugs needs to be taken into account. Thus, the voltage control mode is needed to support glow plugs having a resistance value varying with the temperature.
In an additional mode, the power control mode, the power in the glow plugs is regulated to a predetermined value. Shifts in the resistance of the glow plugs may be compensated because the measured voltage depends on this resistance.
To regulate the power to a predefined value, the power in the glow plug is estimated based on the current through the first switch and based on the voltage at the first switch.
The invention also relates to a method for controlling a glow plug with a glow plug control unit, and an inventive glow plug control unit is provided and the current through the first switch is measured. Then, in a current control mode, the current through the first switch is regulated to a predetermined value.
Preferably, the first switch of the glow plug control unit being provided comprises a transistor. The current through the first switch is measured by a current mirror mirroring the current through the transistor.
The invention also provides a method for calculating the power in a glow plug. First, a glow plug control unit for a plurality of glow plugs is provided. The glow plug control unit comprises a plurality of switches, each of the switches for connecting a glow plug to a power supply node. The current through the glow plugs is measured and the voltage at the glow plugs is calculated by calculating the voltage drop at power supply node based on the current through the switches being switched on concurrently. The power in the glow plugs is calculated based on the calculated voltage at the glow plugs and the measured current through the glow plugs.
If the on-times of the switches party overlap, the voltage drop at the power supply node varies over time. As the number of measurement samples is limited, one sample is used to estimate the voltage of a complete period. When the number of switches being switched on concurrently differs during the period, the voltage drop varies and the sample does not provide the correct value for the complete period. Thus, the voltage drop is calculated based on the number of switches being switched on concurrently to compensate this effect.